1. Technical Field
The present invention relates to a magnetic disk testing method and a surface defect testing device and, particularly, to a magnetic disk testing method and a surface defect testing device with which the throughput of test of magnetic disks for high density recording can be improved.
2. Background Art
A hard disk device (HDD) which is one of external memory devices of a computer uses a hard magnetic disk as its recording medium. The magnetic disk is fabricated by forming a magnetic film on a surface of a circular disk of such as aluminum or glass by painting, and coating the magnetic film with a protective film. The surface of the circular disk having the magnetic film and the protective film thereon is preferably a flat plane having as small irregularity such as protrusions as possible and has a good recording performance. The smoothness of the surface of the magnetic disk is tested by a glide tester and the electric recording performance thereof is tested by a certifier.
The number of tracks of a magnetic disk for high recording density is large and the efficiency of test of the magnetic disk is lowered with increase of the recording density of the magnetic disk. In order to improve the yield and the test efficiency of magnetic disk in the glide tester, it is a recent tendency to provide an optical surface defect test step between a varnishing step and a glide test step. The glide test is performed after defective disks are preliminarily removed by optically testing surface defects of the magnetic disks in the surface defect test step.
FIG. 8 shows a conventional fabrication and test steps of a magnetic disk. First, there is a magnetic film forming step (1) in which a magnetic film and a protective film are formed on a disk of such as aluminum by painting, in the order, and, then, in order to make the irregularity of surface of the magnetic disk having the magnetic film and the protective film formed thereon as small as possible, protrusions on the surface are removed in appearance through a varnishing step (2). Then, the surface defect of the magnetic disk is evaluated in a surface defect test step (3). In the surface defect test step, disks having defects whose size exceeds a predetermined reference value are excluded and, then, tested in a glide test step (4). In the glide test step (4), the existence and the number of protrusions are detected by a glide test head. The glide test step (4) includes a varnishing step for lightly varnishing the magnetic disk. When, as a result of this test, it is found that there are residual protrusions the number of which exceeds a predetermined number, the result of test becomes no-good (NG) and the magnetic disk is returned to the varnishing step of the glide test step (4) to varnish the magnetic disk again. Magnetic disks which become good (G) in the glide test step (4) are processed in a certifying step (5).
Incidentally, an invention in which a glide test and a varnishing step are performed by one and the same device in the glide test step (4) was patented as U.S. Pat. No. 5,423,111.
The surface defect test step mentioned above is intended to improve the yield of magnetic disk in the glide tester and to do not test magnetic disks which may be certainly disqualified in the glide test step. However, the number of magnetic disks which become disqualified in the surface defect test step is practically very small with recent improvement of manufacturing preciseness of magnetic disk. Therefore, the test throughput (test efficiency) per magnetic disk for high density recording is not improved substantially even when such surface defect test step is introduced.